As semiconductor device manufacturers continue to produce smaller devices, the requirements for photomasks used in the fabrication of these devices continue to tighten. Photomasks, also known as reticles or masks, typically consist of substrates that have an absorber layer formed on the substrate. The absorber layer includes a pattern representing a circuit image that may be transferred onto semiconductor wafers in a lithography system. As feature sizes of semiconductor devices decrease, the corresponding circuit images on the photomask also become smaller and more complex. Consequently, the quality of the mask has become one of the most crucial elements in establishing a robust and reliable semiconductor fabrication process.
In order to maintain the quality of the photomask throughout its lifetime, manufacturers have developed a pellicle to protect at least the patterned side of the photomask from being damaged by contaminants that may be present in semiconductor manufacturing tools. The pellicle typically includes a thin film attached to a frame, which has a height that places the thin film outside of the focal plane such that contaminants on the film are not imaged onto a semiconductor wafer. The pellicle frame is typically mounted on the photomask with an annular shaped adhesive gasket that is attached to the bottom and around the perimeter of the pellicle frame.
Due to the opaque nature of the absorber layer, inspecting and controlling the adhesion of the pellicle to the photomask is a difficult task. Advances in pellicle to photomask adhesives have improved the capability of the pellicle mounting process. However, excursions from the normal protocol may not be detected, which can lead to contamination under the pellicle, damage to the membrane, or even damage to the photomask itself.
An accurate measurement of the forces being applied to the photomask is critically necessary to ensure proper adhesion of the pellicle and alert a manufacturer to potential non-conforming products. Furthermore, with the feature size on photomasks and silicon wafers ever decreasing in size, registration or overlay from one pattern layer to the next becomes of the utmost importance. Under some circumstances, the force distribution across the photomask during pellicle mounting can adjust the range of overlay distortion of the photomask by up to approximately thirteen nanometers (13 nm). With wafer fabrication specifications for layer to layer overlay nearing the twenty nanometer (20 nm) range, it is necessary to control the force distribution during the pellicle mounting process in order to prevent the pellicle mounting process from assuming over fifty percent (50%) of the error allowed in overlay measurement. Current industry standard pellicle application tools do not allow the measurement of the force distribution, making controlling, or even understanding the effect of, this force an impossibility.
Pellicle mounting tools currently allow only minor adjustment to the forces applied to the pellicle during the mounting process. Many mounting tools provide the ability to regulate overall, or absolute, force exerted on the pellicle/photomask assembly. However, this regulation is well below the capability needed to accurately monitor and control the force distribution required in the current and future photomask industry. Conventional pellicle mounting tools rely on either a pneumatically driven cylinder, which pushes two parallel fixtures together at even speed and force, or a mechanical drive shaft that moves the pellicle fixtures toward a stationary photomask. While the air cylinder provides a source for equal pressure on the front and back of the pellicle/photomask assembly, the location of the cylinder is well below the needed point of force. This arrangement may result in a radial and uneven distribution of the applied force from top to bottom of the pellicle/photomask assembly. Small variations in setup, fixturing, or milling of the pellicle mounting tool can amplify this uneven distribution, which may cause poor adhesion of the pellicle to the photomask. In current industry standard equipment, this failure cannot be detected without thorough manual inspection or comprehensive mechanical testing of the pellicle frame and adhesive.